Subject: [mobile-ip] Simple Analysis of Mobile IP v4

From: "Subrata Goswami" <[email-masked]>
Date: Fri, 13 Sep 2002 15:15:39 -0700

Submitted a new Internet-Draft.

	Title		: A Simple Analysis of Mobile IP v4.
	Author(s)	: Subrata Goswami
	Filename	: draft-goswami-mobileip-analysis-v4-00.txt

Abstract

This document analyzes Mobile IP v4 from a deployment point of view.
With
the popularity of wireless LAN technologies subnet roaming has become a
prominent issue. Some critical scalabilty issues for Home Agent 
and Foreign Agents are pointed out.

---

From: Gerald Maguire <[email-masked]>
Date: Sat, 14 Sep 2002 18:08:11 +0200 (MET DST)

The number of active tunnels is proportional to the number of
mobiles and the number of active connections that they have (if the
traffic between the HA and the MN is not multiplexed on one tunnel).
Just becase a node may move to a subnet does not meen that there must
always be a tunnel from the HA to this subnet. While there may be
periods of time during which bi-casting or simulcasting of packets may
occur from multiple APs on multiple subnets to a MN - the variance
from proportionality is still small.

Regarding the Home Agent throughput, the HA has to receive and
transmit (at least) once each packet which it is to redirect - thus
the throughput of the HA is at least 2 times the sum of the traffic to
all the MNs which it is a home agent for. However, there is nothing
which prevents you from utilizing multiple home agents and as is well
illustrated by routers it is quite feasible to have a subnets worth of
per host routing tables (even for quite large subnets). Furthermore,
1Gbps is not a problem for the device - but may be a problem for the
links to an from the device if it is not located near a high bandwidth
interconnect point.

Regarding the throughput of the Foreign Agent, an FA only needs to
handle the traffic of the nodes it is service; for simplicity we will
assume that there is one FA on a subnet, thus this FA will never have
to handle more traffic than there can be on the subnet - since the
subnet limits the traffic!

Searching a 64K entry table can be done at evry high speed, witness
high end routers (and encoded structures such as used by the Luleå
algorithm - which supports Gbps routing with a dual processor Pentium
class machine). That the nodes will have to get an address, exchange
messages with the FA -- all the while competing with the other traffic
on this subnet -- I don't think the search time of the table will be
an issue.

Chip

---

From: "s. goswami" <[email-masked]>
Date: Sat, 14 Sep 2002 17:29:50 -0400 (EDT)

See my comments inline.

On Sat, 14 Sep 2002, Gerald Maguire wrote:

> Regarding: draft-goswami-mobileip-analysis-v4-00.txt
>

Yes, it is true that number of active tunnels would be a function of
number of active MN's. Although, you just need one active MN in a foreign
network for an active tunnel.  In the worst case you would have a pair
of tunnels between every subnet.  I am not sure what you are referring
to as " variance from proportionality" - please explain what is
proportional to what.

> links to an from the device if it is not located near a high bandwidth

The measure of throughput depends on whether it is the sum of maximum
input and output or just the maximum output, either one is okay.
I was using the later.

> subnet limits the traffic!

Well, subnets can easily support 100Gbps, you just need 100 Gigabit
Ethernet ports. Each 1 GigE port can support upto 20 802.11 AP's.

> on this subnet -- I don't think the search time of the table will be

Why does the MN need to get an address if the FA is not co-located ?
In a switched Ethernet network, unlike the historic shared medium type
CSMA/CD, not much competition is going on. The Registration Request/Resp
is just two UDP packets and the competition is just access to the port
(duplex) to which the FA is connected.

---

From: Gerald Maguire <[email-masked]>
Date: Sun, 15 Sep 2002 03:04:42 +0200 (MET DST)

> from proportionality is still small.
"s. goswami" <[email-masked]> response:

Tunnels only need to go to MNs, there is no need to have significantly
more (modulo the extra ones that may exist for the time of the handoff
- it is these extra tunnels which may exist due to
simulcasting/bi-casting which which make it not strictly proportional
to the number of mobiles). MNs are unlikely to have active
communication with an unbounded number of other nodes, hence it is
_very_ unlikely that you would need to have tunnels to all subnets.
(Just as it is unlikely that a fixed host has simultaneously
communication with nodes in all possible fixed subnets.)

Unless you have a full-duplex link -- the traffic has to both go to and
leave from an HA and thus is in general 2 times the sum of the traffic
to all the MNs the HA is supporting. Thus even though a MN is in a
subnet with lots of spare capacity, if the mobile's HA does not have
sufficient capacity the MN can potentially suffer a decrease in throughput
This is why HAs that have large numbers of mobile nodes will want to
be near high bandwidth access points.

Why are you going to put significant numbers of APs on an ethernet port?
Most ethernet switches are typically either 8, 12, 16, or 24 ports
with an up link of perhaps 1Gbps - but sadly they don't know about
bi-casting and simulcasting. However, from a parctical point of view,
the current 802.11b and 802.11a devices have a sufficient emitted
power that you will have a hard time putting 20 of them all connected
to the same switched ethernet box - since there are not enough
frequency channels to allow them to be with in the distance
limitations of the copper wires from the ethernet switch to the AP.
Of course you can increase these distances with fiber connections, but
then your prices per port go up considerably. 

Because the ports on the switched ethernet are not significantly
faster than the ports on the router (in fact much of the time they are
the same speed). It is not the registration request/response, but
rather all the AAA traffic which is going to slow down the rate at
which MNs can generate or consume traffic when they arrive in a new
cell. So if there is an FA in the cell responsible for this cell, then
the MN need not get an address, but it still has to negocitate with
this FA for service. Where as if the FA is co-located with the MN, the
MN's FA has to get an address - but J-O Vatn and I showed that this
can be very fast. But following this there is still the matter of
should the subnet provide service to this MN+FA. My key point was that
in comparison to the messaging times the table lookup is small.

Chip

---

From: "Subrata Goswami" <[email-masked]>
Date: Sat, 14 Sep 2002 19:17:52 -0700

Hi Gerald/Chip, thanks for your comments. I am trying to see where I
can incorporate your comments in the draft. In that regard I have
the following questions.

1. Can you please tell me what exactly are bicasting and simulcasting.
2. Why do you need to create tunnels to the MN (if the FA is not
co-located) ?
3. I do not see why you need more than one tunnel to communicate with
one MN.
4. Some situations I have described are possible, however unlikely. If
you have
   100 MN's in a Home Network, each of them potentially can roam into a
different
   foreign network.
5. Why do you not want to connect 25 AP's to a Ethernet port ?
   I have no comment on the economic consequence of doing so, but it is
possible
   technically. 
6. Do you have a number for the  bandwidth consumed by AAA and Mobile IP
registration ?
   I am pretty sure that number is insignificant - say 2 packets of 100
bytes per
   handoff from MN to FA. Then say 4 packets of 200 bytes from FA to
Diameter server. 
   That is 1000 bytes per handoff - 8000 bits - takes about 80 micro
seconds 
   (8000/100,000,000) on Fast Ethernet.   Is it easy to do a lookup in a
table
   of 64K entries in 80 micro seconds ?
7. Moreover the FA that terminates the tunnels has to look up the L2
address of the
   visiting MN for every IP packet tunneled to it. So would not the
lookup process
   be the bottleneck if you have 64K MN roam into one sub-network ?

-----Original Message-----
From: [email-masked]
[mailto:[email-masked]] On Behalf Of Gerald Maguire
Sent: Saturday, September 14, 2002 6:05 PM
To: [email-masked]
Cc: [email-masked]
Subject: Re: [mobile-ip] Simple Analysis of Mobile IP v4

"s. goswami" <[email-masked]> response:
    Yes, it is true that number of active tunnels would be a function of
    number of active MN's. Although, you just need one active MN in a
foreign
    network for an active tunnel.  In the worst case you would have a
pair
    of tunnels between every subnet.  I am not sure what you are
referring
    to as " variance from proportionality" - please explain what is
    proportional to what.

    > links to an from the device if it is not located near a high
bandwidth

    The measure of throughput depends on whether it is the sum of
maximum
    input and output or just the maximum output, either one is okay.
    I was using the later.

sufficient capacity the MN can potentially suffer a decrease in
throughput

    Why does the MN need to get an address if the FA is not co-located ?
    In a switched Ethernet network, unlike the historic shared medium
type
    CSMA/CD, not much competition is going on. The Registration
Request/Resp
    is just two UDP packets and the competition is just access to the
port
    (duplex) to which the FA is connected.

Chip

---

From: "James Kempf" <[email-masked]>
Date: Mon, 16 Sep 2002 08:42:29 -0700

.
>    of 64K entries in 80 micro seconds ?

Doing an AAA transaction from scratch when the mobile hands off is likely to
slow down handover considerably.
Seamboy is working on protocol for context transfer that would allow the old
access point to transfer the AAA
context to the new. Also, the 802.11f (IAPP) specification has some discussion
about this, thought I don't know how
far they have gone in the direction of design.

            jak

---

From: "Subrata Goswami" <[email-masked]>
Date: Tue, 17 Sep 2002 08:11:54 -0700

Hi James, is it in general consensus that involving AAA during handoff
adds latency ?  Is it because the MN has to authenticate and negotiate 
cipher parameters ?  The same for FA can be elimated by maintaining a 
constant session between itself an the RADIUS server.  

The point to note is that in 802.11 networks, the AAA transactions
are made between the MN, the AP and the RADIUS server. So if the 
same AAA transactions are done again for Mobile IP v4 - whatever 
latency AAA adds would be doubled. 

I have heard that 802.11i has made 802.11f (IAPP) redundant - please 
confirm.

Subrata

-----Original Message-----
From: [email-masked]
[mailto:[email-masked]] On Behalf Of James Kempf
Sent: Monday, September 16, 2002 8:42 AM
To: Subrata Goswami; 'Gerald Maguire'
Cc: [email-masked]
Subject: Re: [mobile-ip] Simple Analysis of Mobile IP v4

.
>    of 64K entries in 80 micro seconds ?

Doing an AAA transaction from scratch when the mobile hands off is
likely to
slow down handover considerably.
Seamboy is working on protocol for context transfer that would allow the
old
access point to transfer the AAA
context to the new. Also, the 802.11f (IAPP) specification has some
discussion
about this, thought I don't know how
far they have gone in the direction of design.

            jak

---

From: "James Kempf" <[email-masked]>
Date: Tue, 17 Sep 2002 10:02:27 -0700

> constant session between itself an the RADIUS server.

Of course. Any signaling that is required before the mobile can obtain its
traffic adds latency.

> latency AAA adds would be doubled.

For MIPv6, I don't believe there is any requirement for MIP specific AAA with
the HA. The MN should already have an IPsec SA and therefore no further
authentication should be required.

> I have heard that 802.11i has made 802.11f (IAPP) redundant - please

No idea. You need to contact someone following that work.

            jak